1. Field of the Invention
The present invention relates to phased array antennas. More particularly, the present invention relates to a novel dual-pol notched array architecture having a triangular grid and concentric phase centers.
2. Description of the Related Art
Notch radiating elements for phased array antennas can be designed to support extremely large bandwidths. Notch radiating element designs have been developed that exceed ratios of 9 to 1 bandwidths. One reason for these large bandwidths is that the notch structure acts like a stepped transmission line transformer that matches from free space on to the impedance at a stripline-slotline interface. Typical arrays have a stepped notch transition with three or four stages in the transformer.
For dual polarization (dual-pol), the conventional design is the so-called “egg-crate” architecture, in which the slots are placed on the sides of a square periodic cell. FIG. 1 shows the profile of a typical egg-crate notch section 100 looking into the array. The cross sections 100 in the periodic environment act like transmission lines. Periodic modes (i.e., modes in the infinite array of the notch cross sections 100) have scan and frequency dependent propagation constants and impedances, which can be calculated using a two-dimensional periodic vector finite element code.
One problem with the egg-crate architecture is that the elements are necessarily arranged in a rectangular grid. As a result, a significant greater density of radiators and T/R modules are needed per unit area for a given scan volume relative to the triangular grid of the present invention. In addition, the polarization of the element pattern used in the egg-crate design changes with scan angle. This results from the basic physics of two propagating periodic orthogonal modes that are supported in the notch sections shown in FIG. 1, assuming that the array has been designed to avoid higher order propagating modes in the scan volume.
In an inter-cardinal plane, the notch structure of FIG. 1 has a transverse magnetic (TM) mode, which has a relative propagation constant (kz/k0) equal to 1. However, another mode propagates at a slower rate, (kz/k0)<1. Horizontal or vertical polarization for the element pattern can become circular polarized in the inter-cardinal plane as shown in FIG. 2, which shows the axial ratio from an egg-crate antenna in the inter-cardinal plane. In this example, Phi=45 and frequency was set to 13 GHz. A large value for dB axial ratio corresponds to a linear polarization, whereas a 0 dB value means that the polarization is circular. In this example the polarization is nearly at normal incidence (theta=0), becomes circular for a scan of theta=45 degrees, and tends to linear polarization again as one scans to the horizon (theta=90).
The difficulty with polarization is complicated by the fact that the phase centers for horizontal and vertical polarization are not concentric.
Alternative rectangular architectures have been attempted that consist of concentric notches in a rectangular pattern. One such example is illustrated in FIG. 3. A cross section 300 of the notch transition is shown in FIG. 3 in which the slots 302 are at the corners of a square rectangle.
Such concentric rectangular notched arrays are used with the objective to produce concentric phase centers that coincide for both vertical and horizontal polarizations, to enable easier compensation for changes in polarization. Although the arrangement of rectangular notched arrays is that of a rectangular grid, this architecture has been shown to have significant scan problems for the TE scan in the inter-cardinal plane. Exemplary results from simulation of a full radiator element are shown in FIG. 4. As shown in FIG. 4, the TE scan completely fails at about 25°. This scan failure has been observed both in finite element analysis of periodic arrays as well as measurements of experimental arrays.
The reason for the failure of the concentric fed rectangular array is related to the number and characteristics of the propagating modes in the notch transition. A two dimensional (2-D) periodic finite element analysis of the transmission properties of rectangular concentric notch fins as a periodic transmission line shows three propagating modes. Two modes have a relative propagation constant of kz/k0 equal to 1. One of these two modes always has its electric field in the TM plane. The third mode has kz/k0 less than 1.
In the inter-cardinal plane, the waveguide mode and one of the TEM modes both carry a quadrature piece of the field, which does not radiate well because this field varies faster than the fundamental free space plane wave. This results in poor scan performance.
As an illustration of this behavior, FIG. 5 shows the three propagating modes supported by a periodic transmission line structure consistence of four metal fins per cell. The periodic boundary conditions support scanning off normal to a direction (theta,phi)=(60,30). The fields within a periodic cell are displayed. Each of the six cells in the figure corresponds to the cross section or the radiator periodic cell just above the stripline-slotline transition. Because the array has been scanned to show the undesired behavior, the modes supported by the periodic transmission line structure are fields with real and imaginary components. These are graphically displayed in FIG. 5 by showing the portion in-phase with the field at the center and the portion 90 degrees out of phase (quadrature) at the center. At the stripline-notch transition, the quadrature components in the first and third modes cancel, which can be seen from the direction of the quadrature fields in FIG. 5 (b1) and (b3). The significant result, however, is that as modes 1 and 3 propagate with different propagation constants, the cancellation of the quadrature part between these two modes diminishes because they are no longer synchronized. This quadrature part will not radiate well because it varies more quickly than the fundamental radiated plane wave pair. A similar behavior exists for steps with a wider slot dimension.
Thus, there is a continued need for new and improved radiating architectures that address the above-described problems with prior solutions.